Conventionally, there have been ball-point pens and small-tube type writing implements where a cap with a sealing element therein composed of elastic rubber, etc., to seal the pen point is used when the pen is not used for long periods of time, in order to prevent evaporation of ink and in order to prevent ink starvation which would occur due to drawing of air through the pen point when the pen is impacted by being dropped as well as so-called forward leakage, that is, ink drip from the pen point. However, in the case of a clicking type ball-point pen as well as in the case where the user has forgotten to fit the cap, in the case of a writing implement where a volatile type of ink is necessary, and in other cases, ink starvation and air drawing tend to occur, and the writing implement itself sometimes tends to become disabled. Countermeasures against such cases, include: inhibiting ink evaporation as much as possible by creating the parts of a ball-point pen, using metal; increasing the viscosity of ink (up to about 5,000 cp to 10,000 cp); increasing the content of non-volatile solvent; decreasing the out-flow of ink by narrowing the ink flow passage in the point assembly; and creating an internal pressure in the barrel interior equal to or higher than the pressure of the surrounding air to cause the interior ink to flow out. In particular, these problems have not been resolved for so-called intermediate type ball-point pens which use ink of a medium viscosity and present intermediate properties between oily and aqueous ball-point pens. Although ball-point pens of this type have such imperfections, they have been put onto the market because of other merits. Some ball-point pens which are improved as to these problems have been invented, but no products are yet found which are free from the problem of abrasion in the interior of the point assembly.
In conventional ball-point pens, problems of forward leakage, air drawing and dry-up due to ink evaporation are mainly prevented mechanically, by substantially sealing ink or the interior of the barrel and pressurizing it (with a gas, or by pressing the barrel side face). Alternatively, if no such measures are used, a cap or an elastic material such as rubber etc., for ensuring airtightness are closely fitted on the pen point so as to protect the pen point. When manipulation with a barrel mechanism is used as a measure, the mechanism needs a complicated structure, which causes frequent mechanical accidents and is high in cost, thus increasing the price of the ball-point pen. Products in which the only means for prevention is the cap are not the essential solution because the cap is kept off during actual usage, and they also have the problem of deficiency in maintaining performance if the user does not frequently fit the cap. Further, when ink is manipulated as a countermeasure, a solution is obtained at the sacrifice of the writing performance, such as sluggish drying of drawn lines after writing, uncomfortable writing sensation, increased flow of ink as the pen point becomes abraded by the poor lubrication of the ink as it is used more, and slipping out of the ball or non-writing in the worst case.
The present invention is primarily to provide an improved writing implement of the so-called ball-point pen type wherein a ball projected from its tip as the writing point is loosely held with play inside the point assembly of a popular type and is kept from slipping out. The invention is to provide an inexpensive, high performance writing implement having a point assembly which is able to prevent the problems of dry-up, air drawing, forward leakage, etc., as well as the problem of blotting, i.e., staining of the paper due to adherence of ink to the pen point during writing, without needing a special ink or high-quality barrel mechanism and without compromising the manufacturing performance of the sealing portion in the point assembly having a sealing portion for sealing the interior from the outside air, by the combination of the writing ball and the interior sealing face having a curvature approximately equal to that of the writing ball.